Members especially hollow members 10, 12 such as pipes and pipe fittings one member 12 of cross-linked polyethylene say and the other member 10 of medium density polyethylene say are joined by fusion using a butt fusion machine. End surfaces of the members are pressed at higher pressure against a hot plate 18 to upset only the medium density material to form beads 20, 22. No cross-linked material is upset to form beads. The pressure is reduced during a soak stage then the plate is removed and the surfaces are mutually engaged at the higher pressure. The beads formed are adjacent the interface between the members and both beads are offset towards the medium density material, member 10. Careful alignment will avoid faults caused when the beads do not cover exposed parts of end surfaces. Other materials include polypropylene, copolymer or monopolymer types. Natural gas distribution systems can be fabricated using the invention.
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